The invention relates to a process for the preparation of composite materials which contain small particles of metal or metal oxide in an oxide matrix, the composite materials obtainable by this process and their use as catalysts and in electrical and electronic components.
For many uses, for example in the field of catalysis and in the field of electrical and electronic components, composite materials which contain small particles of metal or metal oxide in oxide matrices (ceramic matrices and ceramic-related matrices) are desirable. These are often achieved by impregnation of ceramic and ceramic-related bodies with metal salts and subsequent reduction thereof to the metal. However, metal particle sizes in the lower nanometer range as a rule cannot be achieved in this way. Moreover, the metal component is applied only to the outer surface of the ceramic material and not into the pores, and the charging of the support with metal is limited.
The sol-gel process on principle offers a solution for preparing composite materials having a high dispersion of the incorporated components, since the starting compounds can be mixed with one another at the molecular level. Several research groups have attempted to prepare composite materials of the type mentioned by the sol-gel process by subjecting silicic acid esters or aluminium nitrate to polycondensation in the presence of metal salts. Metal particles have been produced by subsequent reduction of the gels, but in most cases their diameters do not lie within the lower nanometer range. This method is described, for example, in the following publications: R. A. Roy, R. Roy, Mat. Res. Bull. 19 (1984), 169; R. Roy, S. Komarneni, D. M. Roy, Mat. Res. Soc. Symp. Proc. 21 (1984), 347; G. N. Subbanna, C. N. R. Rao, Mat. Res. Bull. 21 (1986), 1465; and F. Orgaz, H. Rawson, J. Non-Cryst. Solids 82 (1986), 378. The most important disadvantage of this method is that the metal particle size and distribution are scarcely controllable and meet the desired requirements (homogeneous dispersion, very small particle diameter, narrow particle size distribution) only in a few cases.
Ueno et al. (A. Ueno, H. Suzuki, Y Kotera, J. Chem. Soc. Faraday Trans I 79 (1983), 127; K. Tohij, Y. Udagawa, S. Tanabe, A. Ueno, J. Am. Chem. Soc. 106 (1984), 612; H. Tanagawa, K. Oyama, T. Yamaguchi, H. Tanaka, H. Tsuiki, A. Ueno, J. Chem. Soc. Faraday Trans I 83 (1987), 3189; S. Tanabe, T. Ida, M. Suginage, A. Ueno, Y. Kotera, K. Tohij, Y. Udagawa, Chem. Letters (1984), 1567; T. Akiyama, E. Tanigawa, T. Ida, H. Tsuiki, A. Ueno, Chem. Letters (1986), 723; and T. Seiji, F. Koga, S. Tanabe, A. Ueno, Y. Kotera, Nippon Kagaku Kaishi (1984), 998) have reported that composite materials with small and homogeneously distributed metal particles in SiO.sub.2 are obtainable from solutions of metal glycolates and tetraethoxysilane via the sol-gel process after corresponding after-treatment of the gels. The metal glycolates are produced in situ from the corresponding metal nitrates in ethylene glycol. It has been found by EXAFS spectroscopy that after addition of tetraethoxysilane to the solution of the metal glycolate, some glycolate radicals are replaced by OSi(OEt).sub.3 radicals. Linking of the metal ion to the silicate matrix which forms takes place via the alkoxysiloxyl radicals during the polycondensation. Small metal particles are formed by drying the gels and subsequent reduction. The limitation of this method is that composite materials can be prepared only with those metals which form glycolates, that is to say are oxophilic.